In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there have been, and continue to be, efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish higher device density, smaller and smaller features sizes are required. These may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and surface geometry of corners and edges of various features.
High resolution lithographic processes are used to achieve small features. In general, lithography refers to processes for pattern transfer between various media. In lithography for integrated circuit fabrication, a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist. The film is selectively exposed with radiation (such as optical light, x-rays, or an electron beam) through an intervening master template, the mask, forming a particular pattern. Exposed areas of the coating become either more or less soluble than the unexposed areas (depending on the type of coating) in a particular solvent developer. The more soluble areas are removed with the developer in a developing step. The less soluble areas remain on the silicon wafer forming a patterned coating. The pattern corresponds to the image of the mask or its negative. The patterned resist is used in further processing of the silicon wafer. Defects in the mask or the patterned resist may result in defects in the processed wafer.
In spite of advances in lithographic procedures, processed silicon wafers sometimes have defects, such as excess material or particulate contaminants. To improve quality, tools have been developed to detect such defects. One such tool, a scanning probe microscope, will detect many of the types of defects that can occur in processed silicon wafers.
Scanning probe microscopes come in several forms. One form is an Atomic Force Microscope (AFM), which is sometimes alternatively referred to as a Scanning Force Microscope (SFM). AFMs include a sensor with a spring-like cantilever rigidly mounted at one end and having a scanning tip at a free end. AFMs may operate in contacting and non-contacting modes. In the contacting mode, the tip of an AFM is placed in low force contact with a surface of a semiconductor wafer or other workpiece of interest. The workpiece is then displaced relative to the AFM in one or more directions in a plane (e.g., the tip contacts the workpiece in a Z axis while the workpiece is displaced in the X and/or Y directions), to effect a scanning of the workpiece surface. As surface contours or other topographic features are encountered by the tip during scanning, the cantilever deflects. The cantilever deflection is then measured, whereby the topography of the workpiece may be determined.
In non-contacting operation, the tip is held a short distance, typically 5 to 500 Angstroms, from the workpiece surface, and is deflected during scanning by various forces between the workpiece and the tip. Such forces may include magnetic, electrostatic, and van der Waals forces. In both the contacting and non-contacting modes, measurements of a workpiece topography or other characteristic features are obtained through measuring the deflection of the cantilever. Deflection of the cantilever may be measured using precisely aligned optical components coupled to deflection measurement circuitry, although other techniques are sometimes employed.
Another form of scanning probe microscope is a Scanning Tunneling Microscope (STM). Where a feature of interest is non-topographic, AFMs as well as STMs may be used to measure the workpiece feature. Examples of non-topographic features include the detection of variations in conductivity of a semiconductor workpiece material. An AFM can be used to scan a workpiece in the non-contacting mode during which deflections in the cantilever are caused by variations in the workpiece conductivity or other property of interest. The deflections can be measured to provide a measurement of the feature. STMs include a conductive scanning tip which is held in close proximity (within approximately 5 Angstroms) to the workpiece. At this distance, the probability density function of electrons on the tip spatially overlap the probability density function of atoms on the workpiece. Consequently, a tunneling current flows between the workpiece surface and the tip if a suitable bias voltage is applied between the tip and the workpiece. The workpiece and tip are relatively displaced horizontally (in the X and/or Y directions) while the tip is held a constant vertical distance from the workpiece surface. The variations in the current can be measured to determine the changes in the workpiece surface.
In another mode of operation, an STM can be used to measure topography. The scanner moves the tip up and down while scanning in the X and/or Y directions and sensing the tunneling current. The STM attempts to maintain the distance between the tip and the surface constant (through moving the tip vertically in response to measured current fluctuations). The movements of the tip up and down can be correlated to the surface topography profile of a workpiece.
While there has been significant progress in detecting defects, less progress has been made in repairing defects. Parts with defects may be discarded, but it would be advantageous if they could be reliably repaired. With the scaled down geometries of today's semiconductor devices, large wafers can have values, when fully processed, exceeding six figures. There is an unsatisfied need for a system and method of repairing defects in semiconductor devices with small features.